1. Field of the Invention
The present disclosure generally relates to an image forming apparatus that can adjust an image forming condition at a given timing.
2. Description of the Background Art
Typically, image forming apparatuses using electrophotography (e.g., copier, laser beam printer) may need image forming condition adjustment at certain times (e.g., when power is turned ON, after a given time elapses, after a given number of sheets is printed). Image forming condition adjustment may be as follows:
A photoconductor is exposed with a light beam to form latent images while changing an exposing potential for the latent images, and the exposing potential of latent images is detected by a potentiometer; toner patterns are developed on the photoconductor, and detected by an photosensor (hereinafter also “P sensor”); based on detection result obtained by the potentiometer and the concentration sensor, the image forming condition (such as for example exposure power, developing bias voltage, or the like) is adjusted so that the image concentration can be set within a target range constantly.
In such image forming condition adjustment, a characteristic change of latent image potential on the photoconductor relative to the exposure power can be detected, and then a charging voltage and an exposure power can be set to a suitable level by a feedback control using the detection result. Such characteristic change of latent image potential on the photoconductor may be referred to “light attenuation characteristic,” hereinafter.
The light attenuation characteristic of photoconductor may vary depending on such conditions as, for example, use environment, the degree of electrostatic fatigue of the photoconductor, and a thickness of a layer composing the photoconductive layer. Further, in actual apparatuses, several factors that affect the light attenuation characteristic may occur simultaneously or concurrently. Such factors may be abrasion of the surface layer of photoconductor, the degree of electrostatic fatigue of photoconductor, and change of use environment. Accordingly, the light attenuation characteristic of photoconductor may vary under complexed effect of the use environment, the degree of electrostatic fatigue of photoconductor, and photoconductive layer thickness, for example. Accordingly, it becomes difficult to predict a characteristic change of latent image potential formed on the photoconductor relative to the exposure power just based on data of used hours, number of printed sheets, etc. Accordingly, it is important to detect a characteristic change of latent image potential on the photoconductor relative to the exposure power, and to feedback detected results to the image forming condition. Such process may be referred to as an image forming condition adjustment control.
In view of such change of light attenuation characteristic of photoconductor, a related method is described in JP-2004-184583-A for correcting or adjusting an image forming condition.
In such related method, an exposure unit emits a laser beam from a semiconductor laser under a control of a laser control unit. Specifically, the laser beam is emitted by setting a laser emission power at a maximum light intensity. A potential of the photoconductor, which is exposed to such laser beam, is detected by an electrometer, and an output signal of the electrometer is referred to as a residual potential Vr of the photoconductor.
In a normal situation, a potential detected after conducting a charge process, an exposure process, a development process, a transfer process, a cleaning process, and then a de-charging process is referred to as a residual potential Vr.
Because the electrometer is disposed between an exposure unit (or an exposure area) and a development unit (or an development area), a potential after irradiating light having maximum light intensity is referred to as a residual potential Vr instead of the normal residual potential detected after the de-charging process. If the residual potential Vr exceeds a reference value, the difference between the residual potential Vr and the reference value is added to a given charging voltage Vd. A potential obtained by adding the difference to the charging voltage Vd is referred as a target potential.
It is to be noted that the reference value is a residual potential on the photoconductor when the photoconductor is charged by a given charging voltage Vd and then exposed by a light having maximum light intensity under an initial condition.
When forming each of color images, a power supply circuit is adjusted so that a charging voltage Vd for each of photoconductors, applied by a charge unit, can be set to the target potential for each of the photoconductors for each color in parallel.
Further, a laser emission power of a semiconductor laser is adjusted by a laser control unit and then a laser beam is directed onto the photoconductor, which is irradiated by the laser beam, to expose the photoconductor.
Specifically, the laser emission power is adjusted so that an exposure potential, which is a difference between an exposure voltage VL (a surface potential of the photoconductor after exposure) and the target potential can be set to a desirable exposure potential.
A power supply circuit is adjusted to supply a given development bias voltage Vb to a development unit, which includes a black color development unit, a cyan color development unit, a magenta color development unit, and a yellow color development unit, for example. In such adjustment, the power supply circuit is adjusted to supply a given development bias voltage, which can set a desirable development potential between the development bias voltage Vb and the exposure voltage VL.
A description is now given of the conventional correction method for correcting a difference between the residual potential Vr and the reference value.
Firstly, a description is given of the exposure power when the residual potential Vr is measured with reference to FIGS. 1A and 1B.
FIGS. 1A and 1B show relations of the exposure power Lp and the exposure voltage VL when the charging voltage Vd is changed from 600V to 800V to 900V. As shown in FIGS. 1A and 1B, the surface potential of photoconductor reaches a potential saturation point at a given value of the exposure power Lp. The potential saturation point is a condition in which the surface potential of the photoconductor does not substantially change even when the exposure power Lp exceeds a given value.
In FIG. 1A, the surface potential of photoconductor at the potential saturation point changes depending on the charging voltage Vd (i.e., exposure power Lp corresponding to potential saturation point vary depending on values of the charging voltage Vd). By contrast, in FIG. 1B, the surface potential of photoconductor at the potential saturation point does not change greatly between different values of the charging voltage Vd (i.e., exposure power Lp corresponding to potential saturation point does not vary greatly among different values of the charging voltage Vd).
In FIGS. 1A and 1B, the horizontal axis represents an exposure energy (μJ/cm2), and the exposure energy can be read as the exposure power Lp. When measuring the residual potential Vr, an exposure power Lp is set to a given level so that a value of the exposure voltage VL (surface potential of the photoconductor after exposure) does not change when the charging voltage Vd is changed within a given range set for an image forming process. Such given level of exposure power Lp is referred to as an exposure power Lpα, which is not dependent on a charging voltage, which may be referred to as a charging-non-dependent exposure power Lpα, hereinafter.
In case of FIG. 1A, the exposure energy or exposure power Lp is set to 0.35 μJ/cm2 or more, and in case of FIG. 1B, the exposure energy or exposure power Lp is set 0.40 μJ/cm2 or more. Typically, a photoconductor reaches a potential saturation point when the photoconductor is exposed by the charging-non-dependent exposure power Lpα.
A description is now given of the correction method of light attenuation characteristic of photoconductor when the light attenuation characteristic changes due to electrostatic fatigue of photoconductor, with reference to FIG. 2.
FIG. 2 shows the correction method when the light attenuation characteristic changes for the photoconductor of FIG. 1B. In FIG. 2, the exposure power Lp is set to 0.45 μJ/cm2.
Before the electrostatic fatigue of photoconductor (initial condition: see a solid line in FIG. 2), an initial residual potential Vrα has a smaller value, and a good level of exposure potential can be set between the initial residual potential Vrα and an initial charging voltage Vdα (see an initial exposure potential Potα, shown by a solid arrow line in FIG. 2).
After electrostatic fatigue of photoconductor (see a dashed line in FIG. 2), a post-fatigue residual potential Vrβ of the photoconductor becomes higher than the initial residual potential Vrα. Therefore, a post-fatigue exposure potential becomes smaller compared to the initial stage exposure potential (see a post-fatigue exposure potential Potβ shown by a dashed arrow line in FIG. 2).
Accordingly, after such fatigue occurs, an exposure potential which is same as the initial condition can be obtained by increasing the charging voltage Vd, in which the charging voltage Vd is increased for a value computed by an equation of “post-fatigue residual potential Vrβ−initial residual potential Vrα.” Such modified charging voltage is referred to as a corrected charging voltage Vdγ. With such a process, a required exposure potential can be obtained (see a corrected exposure potential Potγ shown by a broken arrow line in FIG. 2).
As such, by correcting the charging voltage Vd, the light attenuation characteristic of photoconductor, which is shown by a dashed curve line in FIG. 2 (a relation of the exposure voltage VL relative to the exposure power Lp), can be obtained, by which a same exposure potential can be obtained for the initial condition and the post-fatigue condition.
When correcting the charging voltage Vd, the residual potential Vr is measured using the charging-non-dependent exposure power Lpα for the following reason.
For the purpose of explanation, a given level of exposure power Lp is set, in which a value of the exposure voltage VL changes when the charging voltage Vd changes. For example, the exposure power Lp is set to 0.15 μJ/cm2 for the purpose of explanation with reference to FIG. 2.
As shown in FIG. 2, even if a photoconductor is exposed with an exposure power set smaller than the charging-non-dependent exposure power Lpα, the post-fatigue exposure voltage VLβ, which is an exposure voltage when the electrostatic fatigue of photoconductor occurs, becomes higher than the initial exposure voltage VLα, which is an exposure voltage at the initial condition, as similar to a relation of the post-fatigue residual potential Vrβ and the initial residual potential Vrα.
Hereinafter, the charging voltage Vd is increased by a value computed by an equation of “post-fatigue stage exposure voltage VLβ−initial exposure voltage VLα” and such increased charging voltage Vd is referred to as a corrected charging voltage Vdδ (Vdδ=Vd+VLβ−VLα). Then, the photoconductor set to the corrected charging voltage Vdδ is exposed by the same exposure power (0.15 μJ/cm2), and an exposure voltage corresponding to such photoconductor is referred to as a corrected exposure voltage VLγ.
The corrected exposure voltage VLγ becomes higher than the post-fatigue stage exposure voltage VLβ. When the corrected exposure voltage VLγ becomes higher than the post-fatigue stage exposure voltage VLβ, a corrected exposure potential “Vdδ-VLγ” becomes smaller than an exposure potential “Vd-VLα” for the initial condition. Accordingly, under an image forming condition using the same exposure power (e.g., 0.15 μJ/cm2), an exposure potential for the post-fatigue stage may not become same as an exposure potential for the initial condition.
However, if the photoconductor is exposed using the charging-non-dependent exposure power Lpα (e.g., 0.45 μJ/cm2), the corrected exposure potential may become same as the post-fatigue residual potential Vrβ, which is an exposure potential before correction.
Accordingly, an exposure potential can be increased for an amount corresponding to an increased value for the charging voltage Vd, by which a required exposure potential can be obtained.
With such a configuration, an exposure potential corresponding to a given exposure power can be set to a level substantially same as the exposure potential at the initial condition. Therefore, when correcting the charging, voltage Vd, the charging-non-dependent exposure power Lpα, which does not change a value of the exposure voltage VL even though the charging voltage Vd changes, must be used.
Further, in the conventional correction method to be described hereinafter, a residual potential Vr that a surface potential of photoconductor becomes saturated may be used to obtain a good level of solid image and halftone image. If the residual potential Vr changes due to a value change of the charging voltage Vd, a suitable correction cannot be conducted. Therefore, to obtain a suitable value of the residual potential Vr, the charging-non-dependent exposure power Lpα may be used.
An image forming apparatus may be used to form the solid image and also the halftone image described above. Accordingly, when the light attenuation characteristic of photoconductor changes, an image forming condition (such as for example exposure power, developing bias voltage, or the like) may need to be adjusted so that the halftone image can also be formed appropriately.
A description is now given of the conventional correction method to obtain a good level of solid image and halftone image.
As above described with reference to FIG. 2, after correcting the charging voltage Vd in view of the photoconductor fatigue or the like, a process control is conducted to compute the exposure power Lp, which can form a good level of solid image and halftone image. This is explained with reference to FIG. 3.
FIG. 3 shows the light attenuation characteristic of photoconductor, exposed for solid image and for halftone image. In FIG. 3, a solid curve line is for the solid image exposure, and a dashed curve line is for the halftone image exposure.
When the halftone image is exposed, an exposure power same as the solid image is used while setting a shorter exposure time per unit area compared to the solid image. Therefore, in the halftone image, each one of exposed dots may have an exposure potential same as the solid image.
However, a potentiometer measures a surface potential of the photoconductor with a given size area, but not each one of exposed dots. Specifically, the potentiometer measures the surface potential of the photoconductor as an average potential value of the given size area. Accordingly, as shown in FIG. 3, even though a same exposing light intensity is used, a halftone image exposure voltage VLh, which is an exposure voltage for halftone image exposure, becomes higher than a solid image exposure voltage VLf, which is an exposure voltage for solid image exposure. Consequently, the halftone image exposure voltage VLh becomes closer to the charging voltage Vd compared to the solid image exposure voltage VLf.
To obtain a good level of solid image and halftone image, the exposure power may be adjusted in view of a desirable photo-induced discharge characteristic. The photo-induced discharge characteristic is defined as an exposure potential ratio (PotB)/(PotA) under a condition that a charging voltage is set at a constant, in which the exposure potential (PotA) is for solid image exposure, and the exposure potential (PotB) is for halftone image exposure. By setting the photo-induced discharge characteristic at a given constant value, a halftone image concentration relative to a solid image concentration can be set in a given range.
In FIG. 3, the photo-induced discharge characteristic of 0.7 is used for adjustment. Further, an exposure duty for solid image may be set to 100%, and an exposure duty for halftone image may be set to 50%, for example.
To obtain a good level of solid image and halftone image, a suitable exposure power Lp is computed based on the halftone image exposure voltage VLh.
At first, the exposure duty of 50% is set (if an apparatus can use pulse number of 64 value for adjustment, the exposure duty of 50% corresponds to 32 value).
Then, when a measurement of the residual potential Vr is conducted, a potential that the photo-induced discharge characteristic becomes 0.7 times of a given reference value for the photo-induced discharge characteristic is defined as a light intensity adjustment target potential Vg.
Specifically, the light intensity adjustment target potential Vg corresponds to an exposure potential PotG that can be computed with an equation of “maximum exposure potential PotM×0.7.” The maximum exposure potential PotM is shown as a solid curved arrow line in FIG. 3, and the PotG is shown as a dashed arrow line in FIG. 3.
As shown as a dashed line in FIG. 3, if the exposure duty is decreased to 50%, a detection result of the halftone image exposure voltage VLh (exposure voltage for exposure duty 50%) may not be saturated, which is different when the residual potential Vr (or the solid image exposure voltage VLf) is measured. Further, when the exposure power Lp is changed, the halftone image exposure voltage VL can be changed, by which the exposure power Lp can be precisely adjusted because the photoconductor has sensitivity in a given area.
The exposure power Lp is adjusted at the exposure duty of 50%, and an exposure power Lp at which the halftone image exposure voltage VLh becomes the light intensity adjustment target potential Vg is computed.
In the example shown in FIG. 3, the exposure power Lp is computed about 0.35 μJ/cm2. Then, the computed exposure power Lp is used to measure the solid image exposure voltage VLf for solid image (exposure duty 100%). Then, a development potential, which is required to obtain a desirable toner adhesion amount, is added to the solid image exposure voltage VLf to determine a development bias voltage Vb. Further, a surface skin potential is added to the development bias voltage Vb to determine the charging voltage Vd.
After determining a suitable exposure light intensity (exposure power Lp) to obtain a suitable solid image and half-tone image when the charging voltage Vd is set at a given value, the solid image exposure voltage VLf computed based on the suitable exposure light intensity has a relation of VLf≈Vr. If the relation of VLf≈Vr is set, even if the charging voltage is computed again as a charging voltage Vd′, a relation of Vd′≈Vd can be obtained. Accordingly, if a suitable exposure light intensity can be set for Vd, such suitable exposure light intensity can be also set for Vd′.
In a case of FIG. 1B, for example, if the residual potential Vr is detected when the exposure power Lp=0.2 μJ/cm2, the residual potential Vr may change greatly due to an effect of a given value of the charging voltage Vd.
If the charging voltage Vd for halftone image control is set to −600V, and the exposure power that corresponds to the photo-induced discharge characteristic of 0.7 for the charging voltage of −600V is 0.15 μJ/cm2, the solid image exposure voltage VLf may become about −250V based on a graph of FIG. 1B, and becomes higher than the residual potential Vr of 200V for about 50V in a negative polarity.
Then, to obtain a desirable exposure potential by conducting a last process for correction process, the charging voltage is corrected by about 50V, by which the charging voltage Vd′=−650V can be obtained.
As such, if the solid image exposure voltage VLf is greatly different from the residual potential Vr, the charging voltage Vd computed based on the residual potential Vr, and the charging voltage Vd′ computed at a last process for correction process based on the solid image exposure voltage VLf becomes greatly different values. Accordingly, the exposure power Lp=0.2 μJ/cm2 may not be a suitable exposure light intensity when to detect the residual potential Vr. Therefore, as for the photoconductor having the light attenuation characteristic shown in FIG. 1B, a greater exposure power (e.g., charging-non-dependent exposure power Lpα) of 0.45 μJ/cm2 may be required as above described. If such a greater exposure power is used for detecting the residual potential Vr when the photo-induced discharge characteristic is 0.7, a suitable exposure light intensity (or exposure power) for a charging voltage Vd of −600 V when detecting the residual potential Vr may become 0.32 μJ/cm2.
Further, as above described, the exposure power Lp may be adjusted so that a relation of “exposure potential×0.7” is obtained under the exposure duty of 50%. If such adjusted exposure power is used under the exposure duty of 100% when to measure the exposure voltage VL, the exposure voltage VL for solid image may become substantially same as the residual potential Vr.
Therefore, if one exposure power, set against an exposure power Lp that can set a surface potential of photoconductor to a saturated-condition, corresponds to a condition of the photo-induced discharge characteristic of 0.7, then the photo-induced discharge characteristic for the exposure duty 50% may become 0.7 times of the photo-induced discharge characteristic for the exposure duty 100%.
When the solid image exposure (or exposure duty 100%) is conducted, a range of exposure power, which may not change a surface potential of photoconductor even if the exposure power is changed a little, is used for an image forming operation. For example, as for the photoconductor of FIG. 3, the range of exposure power may be set from 0.35 μJ/cm2 to 0.43 μJ/cm2 under a condition that the charging voltage is −800V, in which the surface potential of photoconductor changes little if the exposure power changes within such range.
For example, if the exposure power is set to 0.36 μJ/cm2 and than the exposure power changes a little such as for example to 0.35 μJ/cm2, the surface potential of photoconductor changes little as indicated by the profile for the solid image exposure voltage VLf shown in FIG. 3.
Because an image forming process can be conducted using the exposure power having such range, if a solid image exposure is conducted using a suitable exposure power, the exposure potential may not change so much even if the exposure power is changed. In other words, because a sensitivity of the surface potential of photoconductor against a change of the exposure power can be set smaller, the exposure power for solid image exposure may not be adjusted precisely.
Accordingly, the exposure duty is decreased to 50% so that a sensitivity of the surface potential of photoconductor against a change of the exposure power can be set. Because an exposure time can be set to half for a same exposure power, and a light intensity can be set to half, a sensitivity of the surface potential of photoconductor can be set effectively as indicated by VLh. As such, a level of exposure power may be adjusted to a given level.
As such, in a conventional image forming apparatus, the residual potential Vr is detected, and then the exposure power Lp is adjusted based on the detection result of the residual potential Vr. Based on the adjusted exposure power Lp, the development bias voltage Vb and the charging voltage Vd can be computed to conduct an image forming condition adjustment control. With such image forming condition adjustment control, a good level of solid image and halftone image may be formed even if a latent image potential characteristic may change relative to the exposure power applied to the photoconductor.
However, in the conventional image forming condition adjustment control, to detect the residual potential Vr, the exposure power Lp may need to be set to a greater value so that the exposure voltage VL may not change even if the charging voltage Vd is changed, and so that the surface potential of photoconductor after the exposure becomes a saturated condition. In the conventional method, such greater exposure power may be obtained by setting a laser emission power of a semiconductor laser to a maximum value, by which the residual potential Vr may be detected.
However, such high-powered laser emission of the semiconductor laser may not be desirable for the service life or durability of laser and photoconductor. Further, if a linear velocity of photoconductor is increased to attain a higher productivity, a laser output used for detecting the residual potential Vr may also need to be set to a greater value, which is not desirable for the laser and photoconductor from a viewpoint of workload.
Further, a growing market demand for higher productivity and higher image quality (or higher density writing) may need to be addressed. Such demand for higher productivity and higher image quality may be met by rotating a polygon scanner at a higher speed, but such configuration may increase noise level of the polygon scanner, increase electrical power consumption, and decrease durability of the polygon scanner.
Alternatively, such demand for higher productivity and higher image quality may be met by using a light source having multiple beams. For example, Vertical Cavity Surface Emitting Laser (VCSEL) having two-dimensional array may be used as a light source. Such VCSEL can reduce electrical power consumption greatly compared to a conventional end-face emitting laser, and light sources can be integrated in a two-dimensional array with a greater number. Such multiple beam configuration can achieve higher productivity (higher linear velocity of photoconductor) and can reduce the rotation speed of polygon scanner.
However, such plane-emission laser array may have lower emission output, and deteriorate if the emission output is increased. If the emission output becomes smaller, an exposure power such that the exposure voltage VL may not change even when the charging voltage Vd is changed cannot be obtained. Further, an exposure power that can set the surface potential of photoconductor after exposure to a saturated condition cannot be obtained. Such exposure power may not be suitable for an image forming condition adjustment control using the residual potential Vr.